This invention relates generally to semiconductor integrated circuits, and more particularly the invention relates to a laser alignment target for use in such circuits.
Improvements in semiconductor processing technology have led to increased density of circuit components in large scale integrated circuits. The sizes of transistors and other elements, and the spacing of components, are measured in microns.
Large integrated circuits often employ laser blown fuses for circuit programming and for redundancy repair. The individual fuse elements are typically quite small, similar in size to the transistors, in order to make their use cost effective. Accordingly, the semiconductor wafer must be aligned to very tight tolerances when blowing the fuses.
Heretofore, laser alignment targets on the wafers have comprised a bright central feature against a dark background. Transparent oxide layers have been used to provide a dark background since a layer thickness with a half wavelength (i.e. 0.5, 1.5, 2.5 .lambda.) of the laser beam will produce wave cancellation for incident light and result in a dark background. However, oxide thickness cannot be accurately controlled and the thickness can vary from lot to lot in production.
Aluminum on oxide has been used to provide the bright reflective central feature. By depositing the aluminum on a flat oxide surface surrounded by a dimpled oxide surface, the reflective central metal can be surrounded by a light scattering metal due to the undulating metal over the dimples. The light scattering surface then provides a dull background. While this technique has been employed in the prior art, contrast between the light reflective metal and the light scattering metal has not been uniform.
The present invention is directed to overcoming limitations of the prior art.